1. Field of the Invention
The present invention relates to a method and apparatus for the controlled fracturing of a subterranean earth formation. More particularly, the present invention relates to a system for fracturing and creating multiple cracks of fissures in rock and coal stratum or in an oil well using a particular sequence and arrangement of explosive charges to improve production. The present invention can be used for example in oil fields that have known reserves in hydro-carbon-impregnated rock.
2. State of Prior Art
Using an arrangement of charges of different strengths to effect the fracturing of a well bore is generally speaking a known operation. An example of one suggested method is found in U.S. Pat. No. 87,372 issued Mar. 2, 1869 to Taliaferro P. Shaffer. Although it speaks of a main shot which is used in conjunction with two supplemental charges of a lower detonation velocity to blast the fracture, the three charges are simultaneously fired.
Additionally, U.S. Pat. No. 2,921,519 to T. B. Martin issued Jan. 19, 1960 is of interest in that it speaks of a well-fracturing system in which the main shot is boosted by the use of two supplemental shots of a higher detonation velocity. However, it explicitly requires the firing of the supplemental charges before the central charge. In addition the device has a substantially smaller diameter than that of the well hole.
Of further possible general interest to the present invention are the following: U.S. Pat. No. 2,775,940 to Klotz (issued Jan. 1, 1957) which discloses the use of two or more spaced charges for simultaneous firing in a bore hole for enhancing laterally directed explosive forces; U.S. Pat. No. 2,770,312 to Silverman (issued Nov. 13, 1956) which discloses the use of explosives with different detonation velocities, and U.S. Pat. No. 4,018,293 to Keller (issued Apr. 19, 1977) which discloses a use of various shaped charges for effecting the desired result in controlled fracturing.
As will be seen in the preferred embodiment, the present invention not only uses one main shot in combination with two supplemental charges, but inter alia also employs them in a particular sequence and structural array such that the optimal benefit from the combination is realized to an extent never achieved, it is believed, in the prior art.